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Head |
Refers to the height
of a column of water that can be supported by the
pressure or vacuum exerted at the pump.
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Static Suction Head |
The vertical
distance between the pump impeller and the surface
of the liquid on the suction side of the pump.
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Dynamic Suction Head |
The static suction
head plus the additional suction head created by
friction from the liquid flowing through the hoses,
fittings, etc. Atmospheric pressure enables pumps to
lift water. As a result, an atmospheric pressure of
14.7 psi at sea level limits practical dynamic
suction head lift to less than approximately 26 feet
for any pump.
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Static Discharge Head |
The vertical
distance between the pump’s discharge port and the
point of discharge, which is the liquid surface if
the hose is submerged or pumping into the bottom of
a tank.
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Dynamic Discharge Head |
The static discharge
head plus the additional discharge head created by
friction or resistance (usually referred to as
losses) from the liquid flowing through the hoses,
fittings, sprinklers, nozzle, etc.
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Total Head |
The dynamic suction
head plus the dynamic discharge head.
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Pressure |
Pressure is force
per unit area and is usually listed in psi (pounds
per square inch). Pressure is often included in pump
performance curves. Pressure and head are directly
related when referring to pump performance. The
pressure exerted (in psi) at the base of a column of
water is 0.433 x Head (in feet). If you attach
pressure gauge at the base of a pipe 100 feet tall
pipe filled with clear water, you would measure 43.3
psi. Notice how the diameter of the pipe doesn’t
affect the pressure value. The maximum pressure (at
zero discharge) of any pump can be determined by
multiplying the maximum head by 0.433.
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Friction Losses |
The additional
pressure or head created at the pump due to the
friction of the liquid flowing through the hoses,
pipes, fittings, etc. Friction losses always occur
when a liquid is flowing through pipes and becomes
greater as the length of pipe increases and/or the
diameter decreases. Friction losses result in
reduced pump output and can be minimized by used the
largest and shortest hoses possible. Friction losses
are included in dynamic suction and dynamic
discharge head.
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Impeller |
An impeller is a
rotating disk containing vanes coupled to the
engine’s crankshaft. All centrifugal pumps contain
an impeller. The impeller vanes sling liquid outward
through centrifugal force, causing a pressure
change. This pressure change results in liquid
flowing through the pump.
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Volute |
The volute is the
stationary housing enclosing the impeller. The
volute collects and directs the flow of liquid from
the impeller and increases the pressure of the high
velocity water flowing from the vanes of the
impeller.
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Self-Priming |
Most centrifugal
pumps require the pump casing to be filled with
water before starting. Self-priming is a term often
used to describe pumps that have the ability to
purge air from the case and create a partial vacuum,
allowing water begin flowing through the suction
hose. All Honda pumps are defined as self-priming.
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Mechanical seal |
This is a
spring-loaded seal consisting of several parts that
seals the rotating impeller in the pump case and
prevents water from leaking into and damaging the
engine. Mechanical seals are subject to wear when
pumping water containing abrasives and will quickly
overheat if the pump is run without filling the pump
chamber with water before starting the engine. Honda
trash pumps contain silicone carbide mechanical
seals, designed to withstand abrasive conditions.
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Cavitation |
The sudden formation
and collapse of low-pressure vapor (bubbles) across
the vanes of the impeller. When the surface pressure
on a liquid becomes low enough, the liquid will
begin to boil (even at room temperature). With
centrifugal pumps, cavitation can occur when the
suction vacuum becomes to great enough to allow
water vapor or bubbles to begin forming at the
impeller. When this water vapor travels through the
rapid pressure increase across the impeller, a large
amount of energy is released which can cause
impeller damage. Minimizing suction head and using
the largest practical suction hose diameter will
reduce the likelihood of cavitation. You should
never use a suction hose with a diameter smaller
than the pump’s suction port.
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Water Hammer |
Water Hammer is
energy transmitted back to the pump due to the
sudden stoppage of water flowing from the pump.
Water hammer is more likely to occur when using a
very long discharge hose. If the flow of water at
the end of the discharge hose is shut off in less
than the "critical time", energy is transmitted back
to the pump causing a large pressure spike in the
pump housing. Water hammer often results in damage
to the pump casing. |
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For a better understanding of how
pumps work, go to
Pump Theory. |
